The invention relates broadly to a method for sealing containers, and in particular relates to a method for double-sealing a peel-away lid to a container.
Many types of food products, both liquid and solid, are packaged in containers with peel-off lids. Such containers are often designed to hold single portions of items such as condiments and toppings, and are thus referred to as xe2x80x9cportion cups.xe2x80x9d Portion cups are commonly found in restaurants and contain such products as margarine, sour cream, and salad dressings. Solid materials that may be packaged in portion cups include ground pepper and various other seasonings. Portion cups are available in various sizes matched to the normal portion of the materials they are designed to contain. The container itself is generally formed of a plastic material, and may be molded in many shapes, including cylindrical, rectangular, and frustoconical varieties.
Portion cups have become popular because they may be produced relatively inexpensively while offering great convenience to the consumer. Such containers may be easily opened without the need for tools or opening devices. Generally, such containers are formed with a lip or flange extending laterally outward from the open top, to which the lid is sealed with some type of adhesive. The lid generally includes a tab or other handhold portion that extends beyond or above the lip. By gripping the tab and pulling back across the top of the lid, the consumer may easily open the container without spilling its contents.
One of the chief difficulties encountered in designing containers with peel-off lids is the maintenance of a consistent seal strength for all such containers produced. Numerous factors may affect the seal strength for the lids of such containers, such as the temperature at which the seal is made, the pressure applied to the lid during the sealing process, and the particular properties of the adhesive used. When a seal is formed that is too strong, the container becomes difficult to open, and may lead to the consumer using a knife or other potentially dangerous methods of reaching the contents inside. If the seal strength is too weak, then the container may leak during shipment or storage. Rough handling during shipment is a common problem for containers of this sort, as they are most often dropped into large boxes in groups of several hundred with no special padding or packaging to protect them from contact with other containers. If a container spills during shipment under these conditions, the entire box of product may be ruined as the contents of one or a few containers spills onto the other containers. Various environmental factors such as temperature and humidity may weaken seal strength during storage even when the seal strength was within specified tolerances at the time of manufacture. Consumers will reject a container that is not properly sealed, and the defect may reflect poorly on the food vendor that offered the product to the consumer.
Still another problem encountered in sealing containers with peel-away lids is that some portion of the packaged product may be present on the lip of the container prior to seating of the lid during the sealing process. Liquids may splash or spill onto the lip of such containers during filling, and powders or granulated materials may settle onto the lip of such containers in the form of a dust. This problem is exacerbated by the high speed at which the filling operation must proceed in order to keep production costs low. When a lid is applied to a container when such material is present on the container""s lip, the seal in that region of the lip will be poor and the container is far more likely to leak during shipment or storage.
The prior art includes various attempts to improve the durability and consistency of the lid seal for peel-off lid containers. For example, U.S. Pat. No. 5,316,603 to Akazawa et al. teaches a container having xe2x80x9cmicrodentsxe2x80x9d along a sealing layer in the flange or lip portion of the container, or along the sealing portion of the lid. These microdents are arranged in various patterns in the form of dots along the sealing surface between the lip and lid of the container.
U.S. Pat. No. 5,979,748 to Drummond et al. teaches a tubular container with a lid having an adhesive layer; the lid is sealed using heat and pressure. As the lid is pressed onto the lip of the container, the adhesive in the adhesive layer is pressed outward from the center of the lip, forming an outer adhesive bead and inner adhesive bead from the adhesive build-up at the edges of the lip. A thin layer of adhesive may remain between the lid and container lip, or the container and lid may make contact in the center of the lip, such that the entire seal is formed by the inner and outer adhesive beads.
U.S. Pat. No. 4,379,008 to Gross et al. teaches a method for attaching together two pieces of cardboard using a heater with truncated pyramid-shaped projections. One piece of the cardboard has an adhesive layer that contacts the other piece. As the heater is pressed onto the upper piece of cardboard, pressure from the projections causes adhesive to flow outward from beneath the projections and collect in areas between the projections.
U.S. Pat. No. 4,659,405 to Walter teaches a lid and container seal wherein an incompressible rib is formed along the lip of the container. The lid has an adhesive layer that forms a seal with the rib, but the adhesive does not necessarily contact the flat portion of the lip, and no seal with the flat portion of the lip is formed. The rib is sealed to the lid with adhesive, and does not deform during sealing. The pressure applied in this sealing process is not great enough such that the rib pushes through the adhesive layer and comes into contact with the portion of the lid beneath the adhesive layer.
U.S. Pat. No. 4,961,513 to Gossedge et al. teaches a container with a lip having a plastic upper surface and a lid with a plastic lower surface. The lid and container lip are heat sealed together, forming two xe2x80x9cearsxe2x80x9d of material at the outer and inner edges of the seal. The xe2x80x9cearsxe2x80x9d consist of a plastic core formed of material from the lip which is joined to the lip by a thin xe2x80x9croot,xe2x80x9d and an envelope encasing the core formed of material from the lid.
None of the prior art methods described above adequately overcome the problems of forming a consistent seal on a container, particularly when the container may contain overflow of product on the container lip. This problem, particularly acute when solid materials are to be placed in the container, will interfere with each of these sealing methods, resulting in a weak seal when material is present. What is desired is a method for sealing a peel-off lid on a container such that a tight seal is formed even when material may be present on the lip of the container during the sealing process.
The present invention overcomes the limitations of the prior art through a double sealing method to provide a consistent seal even when the lip of the container may be contaminated with overflow product. The container comprises a lid and a cup, with the cup having a lip with a raised sealing bead that runs along its length. The lid includes an adhesive layer on its lower side. The raised sealing bead is compressible, which allows a weld to form between the bead and the lid when the lid is pressed into place, while at the same time an adhesive seal is formed between the lid and the remaining portion of the lip. Thus the lid is sealed to the container both by the weld between the sealing bead and the lid, and by the adhesive bond between the lid and the remaining portion of the lip.
To attach the lid to the container according to the present invention, the lid is positioned over the cup and pressed into place while heat is applied. The heat causes the adhesive to bond the lip to the lid. The raised bead, however, will compress and deform under pressure, thereby stretching adhesive on the lid directly above the bead to either side of the bead, thus allowing the bead and lid to make direct contact. This direct contact is necessary for the weld to form. In addition, the smaller surface area of the bead with respect to the lid as a whole results in much greater pressure along the bead during sealing than would be experienced on the lip if no bead were used. Direct contact between the bead and lid under this high pressure causes the weld between the bead and lid, thereby forming an additional seal between the container and lid.
Any irregularities in the height or shape of the bead will be smoothed during the sealing process as a result of the bead""s compressibility. If the lip of the container should become deformed such that the bead has a high point, that portion of the bead will be subject to correspondingly greater pressure during the sealing process. This higher pressure will cause the bead to compress further at the high point, thereby evening the height of the bead along its length automatically as sealing takes place. In a preferred embodiment, the bead has a relief pocket beneath it that increases the compressibility of the bead during the sealing process, enhancing the ability of the bead to stretch adhesive away from the lid to allow the weld to form, and increasing the capacity of the bead to even itself in height as a result of pressure during sealing.
This double sealing method is particularly well-suited to forming a seal when material has flowed onto the lip of the container. The direct weld between the bead and the lid is along a relatively small surface area of the lip, which reduces the likelihood that any material falling onto the lip would interfere with this bond. More importantly, however, the relatively high pressure between the bead and the lid during sealing will tend to force any material present on the bead to either side of the bead, thereby allowing a firm, solid seal to take place. This is particularly effective with respect to liquid or flowable materials that may have spilled onto the lid, since they may be pushed aside from the bead during sealing. Powders and granulated solids are also pushed aside by the process, thereby allowing a solid welded seal to form.
It is therefore an object of the invention to provide a container with a peel-off lid that has a reduced likelihood of leaking or spilling during shipment or storage.
It is a further object of the invention to provide a method for sealing a container with a peel-off lid that has a double seal between the lid and container lip.
It is a further object of the invention to provide a method for sealing a container that will seal when product is spilled onto the lip of the container prior to sealing.
It is a further object of the invention to provide a method for sealing a container with a sealing bead along its lip that will properly seal when the sealing bead has been warped or deformed prior to sealing.
Other objects of the invention will be evident from the description of the preferred embodiments of the invention and the drawings that follow.